Cold cathode fluorescent lamp (CCFL) is a discharge lamp served in the backlight module of a LCD display panel as a light-emitting unit for the LCD display panel. Generally, the fluorescent lamp are driven by a high-voltage inverter which provides a high-frequency AC voltage to the fluorescent lamp and includes feedback control loops to regulate lamp currents. In a typical LCD display panel, multiple fluorescent lamps are generally required to provide sufficient backlighting for the LCD display panel.
In the application of multiple discharge lamps, the impedance of each discharge lamp is different with one another. Therefore, the current flowing in each discharge lamp is also different with one another. This would not only cause the brightness of the LCD display to become non-uniform, but also shorten the lifetime of the discharge lamp. Eventually, the entire LCD display panel would be damaged.
In order to solve the problem as a result of the current unbalance between discharge lamps, several current balance technique for discharge lamps have been presented to remove this deficiency. As shown in FIG. 1, a primary winding Np1 of a transformer T1 and a primary winding Np2 of a transformer T2 are connected in series with each other, and a secondary winding Ns1 of a transformer T1 and a secondary winding Ns2 of the transformer T2 are respectively coupled with a discharge lamp LP1, LP2. An input DC voltage is converted into a high-frequency AC voltage Vin by a switch device (not shown) and the generated AC voltage Vin is coupled to the primary windings Np1 and Np2. By the transformers T1 and T2, the AC voltage Vin is boosted to a desired AC voltage to drive discharge lamps LP1 and LP2. Because the primary windings Np1 and Np2 both have the same turn number and are connected in series with each other and the turn ratio between the secondary winding Ns1 and the secondary winding Ns2 is 1:1, the currents flowing through the discharge lamps LP1 and LP2 can be balanced. In this case, each primary winding Np1, Np2 will receive a smaller voltage due to the serial connection between the primary winding Np1 and Np2. In order to enable the secondary windings Ns1 and Ns2 to induce an AC voltage of the same voltage level, the turn number of the secondary winding Ns1 and the turn number of the secondary winding Ns2 have to increase. This would disfavor the miniaturization of the inverter and increase the power loss generated in the inverter.
FIG. 2 shows the topology of another conventional power supply for multiple discharge lamps. As shown in FIG. 2, an input DC voltage is converted into a high-frequency AC voltage Vin by a switch device (not shown). The high-frequency AC voltage Vin is coupled to a primary winding Np of a transformer T1 and thus a high-frequency AC voltage is induced across a secondary winding Ns for driving discharge lamps LP1-LP3. The discharge lamps LP1-LP3 are coupled to the secondary winding Ns of the transformer T1 and each discharge lamp LP1, LP2, Lp3 is respectively connected to a choke coil W1, W2, W3. The first windings of the choke coils W1, W2, W3 are coupled with each other and the second windings of the choke coils W1, W2, W3 are coupled with each other. The turn ratio between the first windings of the choke coils W1-W3 is 1:1:1, and the turn ratio between the second windings of the choke coils W1-W3 is also 1:1:1. In this manner, the currents flowing through the discharge lamps LP1-LP3 can be balanced. However, each discharge lamp of FIG. 2 requires a choke coil to be connected therewith. This would increase the number of magnetic elements for use in the inverter and aggrandize the size and cost of the inverter.
FIG. 3 shows the topology of another conventional power supply for multiple discharge lamps, in which a current balance device using common-mode chokes for balancing the currents flowing through the discharge lamps is provided. In FIG. 3, an input AC voltage Vin is coupled to a primary winding Np of a transformer T1 and a boosted high-frequency AC voltage is induced across a secondary winding Ns of the transformer T1 for driving discharge lamps LP1-LP3. A common-mode choke CC1 is coupled with adjacent discharge lamps LP1 and LP2 and a common-mode choke CC2 is coupled with adjacent discharge lamps LP3 and LP2, in which the second winding of the common-mode choke CC1 is connected in series with the first winding of the common-mode choke CC2. With the connection between the common-mode choke CC1 and the common-mode choke CC2, the currents flowing through the discharge lamps LP1-LP3 can be balanced. However, the circuitry of FIG. 3 needs numerous inductive elements, which would increase the number of magnetic elements for use in the inverter and aggrandize the size and cost of the inverter.
There is a need to design a power supply for multiple discharge lamps and the current balance device used in the power supply which is configured to achieve current balance for multiple discharge lamps with a minimum number of magnetic elements and an optimized driving capability for multiple discharge lamps.